The present invention relates to a method for reducing the quantity of data in image coding in which, after discrete transformation (e.g. discrete cosine transformation) of a partial image region into the spectral range or domain, all spectral coefficients are compared with previously determined visual discernibility thresholds, which are adapted to human eye perception, and the comparison result furnishes the information as to which spectral coefficients must be transmitted and which may be neglected.
Since the image information after the comparison for a square partial image of the size of n x n spectral coefficients is composed of a binary word having a length of n.times.n, in order to provide a representation of 2exp [n.times.n]combinations of threshold passages, classification methods have been developed in order to reduce the quantity of over head data which must be transmitted. Such classification causes the quantity of possible combinations to be imaged in, for example 2 exp [3]classes, so as to reduce this additional overhead data quantity from one bit per spectral coefficient to 3/(n.times.n) bits per spectral coefficient.
The problems of very long transmission times which occur during the transmission of individual video pictures over narrowband transmission channels operating with low transmission rates, have evolved many proposals in the literature to reduce these difficulties. For example, an article by H. Lohscheller, "A Subjectively Adapted Image Communication System" IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. Com-32, No. 12, Dec., 1984, pages 1316-1322, discloses a classification method which can be used for this purpose. However, this classification method can be implemented only by performing multiplications, so that the costs for its realization are considerable.